Recent Trends in Modeling of Combustion

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Computational fluid dynamics (CFD) when saw its development on the peak, was meant to merely validate simulations with the experimental results. But the recent trends have shown that confidence in CFD has grown and its use is on the rise to simulate physics problems which have very limited experimental data for its validation. Combustion is one such field where application of CFD and this recent trend has proved to be a boon.

A genre of sophisticated codes for modelling combustion phenomenon has been developed in the last few decades. The challenges faced by engineers and developers is to develop such codes that will represent “perfectly” the observed temperature and concentration profile and to use this codes or models to further simulate and predict the events where no measurements are possible or no data is available. The reader is expected to already have some exposure about CFD modeling and its understanding before proceeding with this article.

To model combustion, the information related to the fluid mechanics of the system must be known. All the transport phenomena such as convection, diffusion as well heat should be accurately incorporated in such models. Chemical reaction schemes must also be known so as to estimate the formation of combustion products and species and also to predict ignition, stabilization and extinction of flames. Radiation also takes place due to presence of soot formed during combustion and also there is radiative heat transfer from wall of the combustion chamber. Combustion of fuels comes under multiphase system, where liquid fuels is a two phase system consisting of liquid and gas phases while solid fuel belongs to a three phase system. The challenges faced are to model breaking up of the liquid fuel, its reaction and distribution of the reactants in three dimensional spatial systems. These models which form part of CFD, has become an indispensable tool for combustion modelling.

Before we begin a detailed review of CFD applied to combustion phenomenon a few points to note for beginners to CFD :

The programs or the models use finite volume methods to solve the Navier-Stokes equations & energy equations.

CFD for combustion in power plant

CFD modelling is used to simulate the combustion of coal in boilers of power plants. Coal undergoes pyrolysis, followed by combustion of coke. Hence the accuracy of the result depends on the sub-models chosen for devolatalisation, char combustion and soot formation. Also heat transfer models and turbulence model, if chosen accurately gives reasonable results. As turbulence affects the heat transfer and chemical processes occurring in pulverised coal boiler, three dimensional flow field is assumed. The transport property of solid fuel is different than the gas surrounding it. Hence for simulation, they are generally modeled as it is and then coupled using a source term in transport equation. Radiation and turbulence find dependence on one another in combustion CFD. Radiation effects come into picture due to fluctuation of temperature and species concentration which are in turn affected due to turbulence. This links up turbulence with radiation.

Below are some of the areas where CFD finds direct application within power plant industry :

Coal burning and combustion analysis.

NOx burner design and analysis of its effect on combustion and thermal performance.

Hydrogen, as a fuel has found its application as a clean source of energy. Hydrogen being a light gas has high transport properties such as thermal conductivity, diffusion coefficient and has wide range of flammability limits. Hydrogen which is used in light water reactor in nuclear power plant when undergoes combustion, results in high pressure loads acting on the container. The pressure loads are a function of the turbulent flame speed. This is where CFD is applied to predict the pressure loads accurately. For this, hydrogen deflagration and its various regimes are identified. While resolving combustion, in which turbulence plays a vital role, the accuracy of the results largely depend on the mesh size and the time step resolution, as well as, on the initial turbulence conditions. This initial turbulence conditions are measured from the experiments which are used to validate the CFD result. Resolving the turbulent flame thickness by the mesh size helps in predicting the results accurately.

Below are some of the application areas :

CFD has application in safety systems designs in below areas :

Hydrogen Management

Boron Dilution Transients

Pressurized Thermal Shock

Thermal Fatigue

Air and water pollution control equipment

Recycling and waste management systems

Chillers, scrubbers and spray towers

CFD analysis of a combustion chamber to model fluid flow

Combustion taking place in a combustion chamber is accompanied with high amount of turbulence. To simulate the piston motion, dynamic mesh is used i.e the mesh generation approach can be used to treat the moving piston as a moving solid body in the computational domain without generating new meshes every time at each crank angle. The finite volume method can accommodate any type of grid. Thus, it is suitable for complex geometries. This fluid flow can be simulated and visualized along with temperature distribution in the combustion chamber.

Below are some of the application areas of CFD within combustion systems :

In space shuttle engines, the combustion chamber has many individual elements which are used to inject fuel and oxidizer in the high pressure chamber. The degree of mixing and the extent of combustion determines the thrust generated. These conditions therefore need to be known accurately. Also practical simulation of these space shuttles is not possible. Hence engineers have largely become dependent on CFD. The distribution of droplets as well as the particle size in the combustion chamber cannot be measured due to presence of many injector elements. Hence investigators have used a simplified way of assuming the fuel to exist in gaseous state, which is sensible as the fuel droplet would enter and evaporate instantly at such high pressures and temperatures. The new spray flame model is computationally efficient for three-dimensional injector flow field simulations.

To summarize this article, we have seen the different areas where combustion modeling is applied as a design tool to come up with new designs within various industries. We also discussed different challenges that one encounters an engineer while applying CFD for simulating combustion. We also reviewed the progress in development of new models for combustion simulation and hope the article threw some light on highlights of combustion modeling and its application within industry.

The Author

Dr. Ganesh is a leading researcher in computational engineering, sustainable energy and its application in process industry. He has done immence contribution in LearnCAx. Before he joined post-doctoral fellowship at IIT Delhi, he was the general manager at LearnCAx. He was instrumental in conceptualization, development and implementation of online education from CCTech for CAx professional. Ganesh has a number of publications both in international journal and conference proceedings. Before CCTech, he held the position of Associate Scientist in solar thermal division at Sardar Patel Renewable Energy Research Institute, Anand.